[patch 06/11] psi: get poll_work to run when calling poll syscall next time

[akpm@linux-foundation.org]

From: Jason Xing <kerneljasonxing@linux.alibaba.com>
Subject: psi: get poll_work to run when calling poll syscall next time

Only when calling the poll syscall the first time can user receive POLLPRI
correctly.  After that, user always fails to acquire the event signal.

Reproduce case:
1. Get the monitor code in Documentation/accounting/psi.txt
2. Run it, and wait for the event triggered.
3. Kill and restart the process.

The question is why we can end up with poll_scheduled = 1 but the work not
running (which would reset it to 0).  And the answer is because the
scheduling side sees group->poll_kworker under RCU protection and then
schedules it, but here we cancel the work and destroy the worker.  The
cancel needs to pair with resetting the poll_scheduled flag.

Link: http://lkml.kernel.org/r/1566357985-97781-1-git-send-email-joseph.qi@linux.alibaba.com
Signed-off-by: Jason Xing <kerneljasonxing@linux.alibaba.com>
Signed-off-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: Caspar Zhang <caspar@linux.alibaba.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 kernel/sched/psi.c |    8 ++++++++
 1 file changed, 8 insertions(+)

--- a/kernel/sched/psi.c~psi-get-poll_work-to-run-when-calling-poll-syscall-next-time
+++ a/kernel/sched/psi.c
@@ -1131,7 +1131,15 @@ static void psi_trigger_destroy(struct k
 	 * deadlock while waiting for psi_poll_work to acquire trigger_lock
 	 */
 	if (kworker_to_destroy) {
+		/*
+		 * After the RCU grace period has expired, the worker
+		 * can no longer be found through group->poll_kworker.
+		 * But it might have been already scheduled before
+		 * that - deschedule it cleanly before destroying it.
+		 */
 		kthread_cancel_delayed_work_sync(&group->poll_work);
+		atomic_set(&group->poll_scheduled, 0);
+
 		kthread_destroy_worker(kworker_to_destroy);
 	}
 	kfree(t);
_